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A study in the journal PLOS ONE uses volunteer diver surveys to assess the impacts of sea star wasting disease in the Salish Sea. Data shows that sunflower sea stars were especially hard hit and have all but disappeared from the region.

Competition occurs when individuals of different species struggle to obtain the same resource in an ecosystem (such as food or living space). Adaptations, such as physical mutations and behavior modifications, can help an organism outcompete its competitors.

Biology/Natural History: This species of seastar is often considered a keystone species in many intertidal regions. P. ochraceus feeds mainly on mussels (especially Mytilus californianus and Mytilus trossulus) or will also feed on barnacles, snails, limpets, and chitons when mussels are absent. P. ochraceus will insert its stomach into snail shells or slits as narrow as 0.1 mm between the shells of bivalves. Numerous species of mollusks have avoidance responses to the Ochre Sea Star, often involving moving away. Adult ochre seastars have few predators, but may be eaten by sea otters and sea gulls. P. ochraceus is more tolerant to air exposure than others in the Pisaster genus and regularly withstands up to 8 hours exposure during low tides. It is apparently unharmed by up to 50 hours of exposure in laboratory setting; but they have an inability to tolerate high water temperatures and low oxygen levels, keeping them out of shallow bays and high tidepools (See Pincebourde et al., 2008). Sexual reproduction occurs in the late spring or in the early summer. When ready to reproduce, mature gonads may account for up to 40 percent of the animal's weight. Spawning occurs in the Puget Sound around May to July. Fertilization occurs in the sea and development results in free-swimming, plankton-feeding larvae. Embryonic development and larval feeding have been studied in detail, however little is known of juvenile life following settlement and metamorphosis. P. ochraceus has been the focus of many major studies including tests on their digestive gland tissue (which is similar to cells in the mammalian pancreas and secretes materials similar to insulin).

Pisaster ochraceous is less water permeable than some other intertidal species such as Pycnopodia helianthoides. It makes extensive use of water intake through its madreporite to maintain internal fluid balance (Ferguson, 1994). The species is still highly susceptible to osmotic changes, however. Held and Harley (2009) studied populations from high and low salinity sites. Individuals from both populations were almost complete osmoconformers over the range of 15 to 30 psu. In both populations activity (as measured by the righting response) was lowest at the lowest salinity (15 psu), and the population which had been living at lower salinity did not have any better righting response than did the one living at high salinity. The population living at high salinity, however, did experience a higher mortality after exposure to 15 psu than did the other population. Feeding rates on mussels also varied with salinity, but the maximum feeding rate in the population living at low salinity was at a lower salinity than that of the population which lived at a higher salinity.

This species of seastar has a radius of up to about 25 cm with stout rays that taper towards the end. There are usually five rays, but occasionally as few as 4 or rarely as many as 7. The color ranges from pale orange to dark brown or deep purple (photo). The aboral surface contains many small spines (ossicles) that are arranged in a netlike or pentagonal pattern on the central disk (photo).

Pisaster ochraceous can be found anywhere from Alaska to Baja California. It is most commonly found in the Northeastern Pacific, being that it is a cold-water species. However, it is common in bays all year.

Biogeographic Regions: pacific ocean

References:

Banister, K., A. Campbell. 1985. The Encyclopedia of Aquatic Life. New York: Facts of File Publications.

Meinkoth, N. 1981. The Audubon Society Field Guide to North American Seashore Creatures. New York: Chanticleer Press, INC.

The radius of P. ochraceus is anywhere from about 10 to 18 inches (25 to 45 cm) in diameter. Yellow, orange, brown, reddish or even purple make up the ranging colors of this heavy starfish. Basically it has a good size middle disk with five stout arms sticking out. On its upper side it has short, white spines in the pentagonal pattern.

How to Distinguish from Similar Species: Troschel’s Sea Star (Evasterias troschelii) may be confused with P. ochraceus at times. E. troschelii is distinguished from P. ochraceus by the smaller disk size and longer, tapering rays which are often thickest a short distance out from the base rather than at the base as in P. ochraceus; clusters of pedicellariae among the spines that border the ambulacral grooves, and the absence of a stellate pattern of spines on the aboral surface of the disk. There are two other, mostly subtidal, local species of Pisaster (Pisaster giganteus and Pisaster brevispinus) but they have different aboral spines and coloration which allows one to distinguish between the species.

Pisaster ochraceous can be found on wave-washed rocky shores, from above the low-tide zone to 90 m in depth. Because they can live in shallow water they need to survive in these living conditions, including strong surges, big temperature changes, dilution by rainfall, and dessication. Pisaster ochraceous is very resistant to dessication and it can tolerate a loss of thirty-percent of its body weight in body fluids.

Like all sea stars, an adult P. ochraceus has tube feet which they use for locomotion and for handling prey. Pisaster ochraceous feeds on mussels, chitons, and limpets, which they slowly pry open and devour. Snails, barnacles, echinoids, even decapod crustacea are also eaten. Pisaster ochraceous everts its stomach over the prey if it is too large to be swallowed whole, and digests the prey before swallowing it.

Pisaster ochraceous is a predator and is a prey to sea otters and sea gulls. Its role as a keystone species has been well studied. In intertidal areas of Washington, when it was removed, the diversity of species in the area decreased.

Sea otters and gulls prey on this starfish. Pisaster ochraceus can retract such sensitive areas as the podia and skin papillae. Additionally, they may be able to shut the ambulacral grooves which contain the tube-feet, and then spread the spines over them protectively.

Sea Star Wasting Syndrome

Pisaster ochraceus is the first sea star species observed with “starfish wasting syndrome,” a plague of significant concern to scientists, which causes sea stars along the North American Pacific coast to soften, lose their arms, and eventually disintegrate, sometimes just days after first symptoms appear. The outbreak is similar die-offs in 1970 and in 1990, however far more geographically expansive and affecting far more individuals. Scientists first sighted symptoms of the syndrome in September 2013 on sea stars on the Olympic Penninsula in Washington state; since then, massive population declines of many sea star species have been documented along much of the North American west coast, and continue to erupt in previously unaffected areas.

Known as a “keystone species” in the marine intertidal and subtidal ecosystems, the sharp population decline of P. ochraceus is expected to trigger wide-ranging changes in the relative compositions of other species, and cause a fundamental disruption at ecosystem level. Scientists still do not know the underlying cause for the wasting syndrome, and are examining potential infectious agents and environmental changes that might be implicated.

The neurosensory cells scattered over the asteroid body respond to mechanical, chemical, and optical stimuli. Sensory organs are developed only at the base of each terminal tentacle. At this location a great number of light-sensitive cells form an optic cushion which contains several ocelli.

Pisaster ochraceous develops through several larval stages, one including the brachiolaria larva. Using ciliated arms to sweep food into its mouth, it glides through the water column. The cilia drive locomotion of the larva is supplemented by these same arms. The larva attaches itself to the substratum as it settles because each arm has a glandular tip. The five-armed adult is formed because it undergoes metamorphosis. Adults continue growing and the rate of growth is dependent on its food supply.

Reproduction

Pisaster ochraceous is mainly dioecious. The male gametes develop, but later only females ones are produced. During a transitional period, both eggs and sperm are produced. A pair of gonads branches into each arm off a circular genital strand located along the oral inner surface of the disc. Each gonad looks like a feathery cluster of tubules. During maturation of the gametes, the gonads greatly increase in size, pushing into the perivisceral cavity of the arms, often right up to the ends of the arms. The gonopores of the individual gonads open at the bases of the arms.